Study On The Effect Of Impurity Gallium On The Microzone Corrosion Behavior Of Al Alloy Protective Film Based On SECM

Al alloys are widely used in transportation,shipbuilding,aerospace,and other fields due to their high specific strength,corrosion resistance,and good formability.Ga is a common impurity in electrolytic aluminum,which is generally between 50 and 150 ppm.The influence of impurity Ga on Al alloys is mainly to activate aluminum alloy,reduce its corrosion resistance and affect the service life of Al alloys.Anodic aluminum oxide（AAO）film and zirconium-titanium conversion coating on the Al alloy are common surface treatment technologies to improve its corrosion resistance.The impurity Ga in the Al substrate may affect the microstructure and properties of surface protective film.However,there are few studies on the effect of Ga on the microstructure and corrosion behavior of AAO film and zirconium-titanium conversion film.The corrosion failure of the protective film on the Al alloys usually starts with the localized corrosion,but the traditional electrochemical method（EIS or Tafel）only provides the macroscopic average corrosion information of the whole film,and cannot provide the real-time change information of the micro-area corrosion of the protective film.Scanning electrochemical microscopy（SECM）has high spatial resolution and electrochemical sensitivity,which can in situ characterize the micro-area corrosion behavior of protective film on the Al alloy surface.Therefore,in this work,the effect of impurity Ga on the growth behavior of AAO films and zirconium titanium conversion coatings was studied by XRD,SEM-EDS or XPS,and the influence of impurity Ga on the micro-area corrosion behavior of AAO film and zirconium titanium conversion coating on the Al alloy was investigated in situ using the SECM.The main conclusions are as follows:（1）The corrosion potential of pure Al decreases from-0.715 V to-1.005 V with the impurity Ga content increases from 0 wt.%to 0.5 wt.%,and the impedance value decreases from 4.78 KΩto 0.419 KΩ.At the same time,the occurrence time of pitting is significantly shortened,and the number of pitting is increased.This is mainly because the dissolved Ga³⁺in Al can deposit again on the Al surface,damaging the Al₂O₃ film at the deposition site and making it more prone to electrochemical corrosion.（2）Under constant current anodizing,the increase of impurity Ga content reduces the anodization voltage of Al,resulting in the decrease in the resistance of anodization,and the AAO film grows thicker at the same time.The impurity Ga in the Al substrate promote the dissolution of the pore walls of the porous layer,thereby increasing the pore size of the AAO film.The electrochemical impedance of the AAO film is negatively correlated with the content of the Ga content,while the number of surface pitting on the AAO film is positively correlated with the Ga content,which means that the impurity Ga reduce the protective ability of the AAO film.（3）The zirconium-titanium conversion coating is mainly composed of cryolite,oxides（such as Ti O₂,Zr O₂,Al₂O₃）,and organic complexes.The outer cryolite is coarse and the inner cryolite is fine in the Zr/Ti coatings.When the Ga content exceeds 0.02 wt.%,the whole film is fine cryolite.The impurity Ga has an inhibitory effect on the growth of zirconium-titanium conversion coatings.When impurity Ga is present in Al substrate,there are many defects in the zirconium-titanium conversion coating,which results in incomplete coverage of the Al substrate and thinning of the conversion coating.With the increase of Ga content,the corrosion resistance of zirconium-titanium conversion coating continuously decreases.The SECM current distribution shows that the zirconium-titanium conversion coating is more prone to local corrosion damage with the Ga content in Al exceeds 0.02 wt.%.（4）After the zirconium-titanium conversion coating is damaged,the nearby conversion coating dissolves and redeposits at the damage to form the self-healing layer,which plays a role in self-healing and protecting the substrate.However,the impurity Ga can inhibit the self-healing behavior,resulting in the location of Ga enrichment is not easy to appear self-healing film,and the secondary protection of the Al substrate is limited.When the Ga content in Al reaches 0.05 wt.%,the protective performance of the new film formed by self-healing begins to weaken.While the Ga content exceeds 0.1 wt.%,the protective effect of the self-healing basically disappears.